The present invention relates to synthetic polymers and, in particular, to substituted cellulose acetates, and methods of use thereof.
Cellulose acetate is an acetate ester derivative of cellulose, a naturally occurring biopolymer comprising β-D-glucose monomer units, and is widely used to make a variety of consumer products. Naturally occurring cellulose is insoluble in water and most organic solvents. However, the three free hydroxyl groups of each glucose monomer unit in cellulose can be derivatized to modify certain properties, for example, solubility in certain solvents. Cellulose acetate is thought to be among the most commercially useful derivatives of cellulose, and its specific physical and chemical properties generally depend largely on the degree of substitution of acetate on the three free hydroxyl groups of a glucose monomer unit.
Since its first synthesis, cellulose acetate has been used, among other things, as a film base in photography, a component in adhesives, and a raw material used in the manufacture of cigarette filters. One of the key attributes of cellulose acetate is that it can be processed into many different shapes and forms, including, films, tows, flakes, fibers, and solids, depending on the desirability of the application. For example, cellulose acetate tow is primarily used in the manufacture of cigarette filters.
Most typically, derivatization of cellulose is conducted using acidic catalysts at elevated reaction temperatures. The synthesis of cellulose acetate involves acetylating cellulose, which is commonly obtained from wood pulp, in the presence of acetic anhydride and acids. Acetic acid and sulfuric acid are the two acids usually present during the acetylation reaction. In this reaction, the acetic anhydride serves as the primary acetylating agent; the acetic acid is used primarily as a diluent; and a small amount of the sulfuric acid is used as the catalyst. In general, the monomeric units of cellulose have three hydroxyl groups that are readily available for substitution, e.g., via acetylation. The monomeric units at both ends also have an additional hydroxyl group that are typically involved inter-sugar linkages.
The product of the acetylation reaction is a primary cellulose acetate that usually contains high concentration levels of acetate or, in other words, has a high degree of substitution (DS) of acetate. As used herein, “degree of substitution of acetate” generally refers to the average number of acetates per glucose monomer unit. The structure of cellulose acetate may be generally characterized by specifying the degree of substitution of acetate. Exhaustively acetylated cellulose is commonly referred to as cellulose triacetate, where, according to Federal Trade Commission guidelines, at least 92% of the hydroxyl groups are substituted with acetyl groups. For example, when there are at least about two acetyl groups per cellulose monomer unit (that is, a DS of about 2), the acetyl cellulose can become significantly less biodegradable until at least some of the acetyl groups are removed via chemical or enzymatic hydrolysis. Acetylated cellulose having reduced DS of acetate can be prepared by controlled hydrolysis of cellulose triacetate.
Next, the primary cellulose acetate is hydrolyzed in the presence of acetic acid and sulfuric acid to reduce the DS of acetate of the primary cellulose acetate. The resulting hydrolysis product is usually a cellulose acetate flake having a DS on the order of about 2.4 to about 2.6.
Once cellulose acetate flake is obtained, it may be subjected to further treatment in order to process the cellulose acetate to its desired form. For example, cellulose acetate tow may be formed by dissolving the cellulose acetate flakes in acetone, putting cellulose acetate solution through a series of filtration processes, and dry spinning the acetone dope through microscopic holes in a spinneret. As the cellulose acetate moves through this extrusion process, filaments of cellulose acetate are collected. These filaments are typically bundled and then crimped to ensure tow band cohesion as well as desired bulk. The end result is a cellulose acetate tow, which is essentially a continuous band composed of several thousand filaments held loosely together by crimp. The cellulose acetate tow filaments usually are very thin and may be packed together tightly to create a filter. As used herein, “cellulose acetate tow” generally refers to post-synthesis processed bundles of filaments of cellulose acetate.
Cellulose acetate is also environmentally friendly in that it is a readily degradable material made largely from a renewable source, wood pulp. While cellulose acetates that vary in their DS of acetate are structurally similar, this variation can have some effect on the biodegradability of the cellulose acetate. At higher degrees of acetyl substitution, the rate of biodegradation can be significantly reduced relative to naturally occurring cellulose or cellulose having less acetyl substitution. Cellulose acetates having a lower DS of acetate biodegrade more readily than cellulose acetates having a higher DS of acetate. This trend is believed to taper off at lower DS of acetate values because the material becomes insoluble in water. In one example, the biodegradation rates of cellulose acetate having DS values of 1.85, 2.07, and 2.57 were tested for their biodegradability (FIG. 1). The cellulose acetate with a DS of 1.85 showed the highest biodegradation rate while the cellulose acetate with an acetyl value of 2.57 showed the slowest biodegradation rate.
While cellulose acetate is a versatile composition with many commercial uses, the properties of certain cellulose acetate derivatives have not been exhaustively studied. For example, in a typical synthesis, residual sulfate groups are typically removed in order to improve thermal stability. As a result, cellulose acetates synthesized by typical means will have about 0.005% or less of sulfur atoms by weight. As such, there are potential derivatives of cellulose acetate that may provide enhanced properties for many of the consumer applications of cellulose acetate.